Wide attention has been given to metallocene catalysts for their ability to make ethylene polymers having relatively narrow molecular weight and comonomer distributions at excellent polymerization rates. Such narrow molecular weight and comonomer distributions contribute to improvements in clarity, toughness, and extractables levels in ethylene polymers having densities much below 0.95 g/cc. However, for some applications requiring processing ease, such as enhanced extrudability, these ethylene polymers can be deficient due to their narrow molecular weight distributions. For example, U.S. Pat. Nos. 5,420,220 and 5,324,800 disclose metallocene-made, linear low-density polyethylenes having characteristic narrow molecular weight and comonomer distributions, along with the associated limitations in processability.
Unfortunately, if the molecular weight distribution of an ethylene polymer is broadened in order to improve processability, the clarity and impact strength of the polymer diminish. In addition, extractables increase, especially for ethylene polymers having densities much below 0.93 g/cc. To improve the processability of an ethylene polymer while maintaining a narrow molecular weight distribution, long chain branching may be incorporated into the polymer. For instance, U.S. Pat. Nos. 5,272,236 and 5,278,272 and PCT Application No. WO94/07930 describe metallocene-made, very low-density and low-density polyethylene having long chain branch structures that are reported to have improved processability. However, long chain branch structures sometimes promote directional orientation during fabrication leading to an imbalance in mechanical properties and reduced impact and tear resistance. The clarity of fabricated articles such as blown film may also be less than optimum for long chain branched ethylene polymers even with narrow molecular weight and comonomer distributions. It would be advantageous, therefore to identify ethylene polymers having the benefit of long chain branching that maintain good mechanical strength and clarity.
U.S. application Ser. No. 08/611,278 filed on Mar. 3, 1996, now abandoned discloses ethylene polymers having narrow molecular weight and comonomer distributions, and very high clarity. However, these polymers do not possess a high degree of fabrication ease, i.e., extrudability, bubble stability in blown film processing, and melt instability resistance in cast film processing. This is due to their relatively low normalized Relaxation Spectrum Index (nRSI), which ranges from about 2.5 to about 6.5. U.S. Pat. No. 5,527,752 generically discloses catalyst compositions that are complexes of transition metals, substituted or unsubstituted .pi.-bonded ligands, and heteroallyl moieties, which may be used to make these ethylene polymers.
An ethylene polymer having fabrication ease superior to that of the ethylene polymer disclosed in U.S. application Ser. No. 08/611,278 filed on Mar. 19, 1996, now abandoned and yet maintaining good mechanical strength and clarity, has now been discovered. This ethylene polymer has a broadened relaxation time distribution that is defined by its high nRSI and cRSI.